The types of gene structural changes causing deficiency of hypoxanthine guanine phosphoribosyl transferase (HGPRT) activity in spontaneous mutations is being examined in cultured human fibroblasts. The deficiency of this enzyme activity causes a human disease (Lesch-Nyhan Syndrome). The restriction enzyme cleavage patterns of HPRT gene sequences in mutant lines will be analyzed. The work is presently focused on obtaining a large number of independent spontaneous mutants that existed in newborn baby foreskins. Ten independent mutants have been isolated from different human subjects. The mutant cells were grown to large numbers. Portions of cultured cells were frozen in liquid nitrogen for cytogenetic and enzymology studies at a future time; while portions of cultured cells were frozen for DNA extraction. Southern blot analysis is now in progress to assess the possible involvement of DNA rearrangements in spontaneous mutation.